The present invention relates to a PTC (Positive Temperature Coefficient) thermistor element and more particularly to a PTC element used to protect against electrical circuit overcurrent surges.
Conventional PTC elements used to protect an electrical circuit use polymer dispersed carbonaceous conductive particles for PTC properties and a metal electrode affixed to the polymer. Polyethylene is conventionally used for the polymer component. Electrical stability is difficult to attain with these PTC elements, however, because the difficulty of joining or attaching the metal electrode to the polyethylene with sufficient bonding strength makes the resulting bond unpredictable. A second major drawback of these PTC elements is their tendency to peel during repeated use. This peeling is due to a difference in the coefficients of thermal expansion between the metal and polyethylene.
A further problem with PTC elements of the prior art is the fact that polyethylene is slightly permeable to gases, and the metal electrodes are impermeable. Thus, gases attempting to escape the polyethylene may collect under the metal electrodes, and encourage degradation of the bond.
Many methods for overcoming these problems have been used. For example, Japanese Patent Laid-Open No. 38162/1982 discloses a method wherein the surface of an electrode is treated with a titanate coupling agent where it is joined to the PTC element. The electrode is then bonded to the PTC element by thermal compression.
For another example, Japanese Patent Laid-Open No. 196901/1985 discloses a polymeric PTC thermistor wherein, prior to bonding, a surface of an electrode is roughened at the point where it joins the PTC element. The roughened surface contributes to mechanical keying, and thus improves the bond.
In yet another example, Japanese Patent Laid-Open No. 229679/1987 discloses a resistor composed of resin and conductive particles whose electrode is one of the following:
a low resistance compound produced by blending conductive particles in the same resin as the resistor, or in a resin capable of thermal fusion with the resistor;
a metal or carbon fiber coated with the low resistant compound.
Further, Japanese Patent Laid-Open No. 265401/1988 discloses a polymeric PTC thermistor using carbon fiber or activated carbon fiber as its electrode.
However, attaching a metal leaf electrode firmly to a conventional polyethylene PTC element remains problematic, and attaining electrical stability remains uncertain.
PTC elements that use metal electrodes have still another drawback. The electrodes of these PTC elements tend to peel during and after a thermal shock.
A metal electrode presents yet another problem. During cross-linking by gamma ray irradiation after attachment to a PTC element, an electrode may trap decomposition gas from the PTC element. This tends to destroy the bond.
Japanese Patent Laid-Open No. 229679/1987 discloses a PTC element, that consists of carbonaceous conductive particles and a polyethylene polymer. This PTC element is used with an organic electrode consisting of the same resin and conductive particles as the PTC element. This approach yields sufficient adhesion, but the use of similar resins for both the PTC element and the electrode causes other problems.
The resin composition of the PTC element is designed to open or trip at a predetermined temperature to protect an electronic circuit. Because the electrodes are formed of the same PTC composition as the PTC element, they are subject to thermal deterioration as they rise in temperature. As a result, these electrodes can fail at temperatures lower than the designed tripping temperature of the PTC element.
Because carbonaceous conductive particles are used for the organic electrode, the electrical resistance of the electrodes is high relative to a metal electrode. A commonly used conductive carbon black is Ketjen black. Although Ketjen black has a volume resistivity of about 1 ohm.cm, at a minimum, the volume resistivity of the electrode is considerably higher than this value. If the ratio of carbon black in the electrode is increased to a significant degree in an attempt to reduce the volume resistivity of the electrode, the composition of the electrode is weakened to the point where it is no longer usable.
Another problem with organic electrodes is that they cannot be attached to metal holders. This is not a problem with, for example, metal electrodes.
Yet another problem is that a polymer having a low affinity with the crystalline polymer used in the PTC element cannot be used for an organic electrode.